1. Field of the Invention
The present invention relates to key telephone systems, and in particular, to operative connection of separate key telephone systems.
2. Description of the Prior Art
Key telephone systems of a type are known in the prior art wherein respective key telephone sets are connected to a main equipment through connecting cables, as disclosed in, for example, U.S. Pat. No. 4,347,409 by Ogawa et al, assigned to Nippon Telegraph & Telephone Public Corporation and Nippon Tsushin Kogyo Kabushiki Kaisha.
In the known key telephone system, the main equipment comprises at least one office line interface circuit connected to the office line, a plurality of key telephone interface circuits connected to key telephone sets, respectively, a switching such as office line selection keys, an interextension call key, and others, and a set of dialling keys or ten keys.
In the key telephone system, a speech path can be established through the main equipment between any one of key telephones and another one by operation of the interextension key and the ten keys at the any one of key telephones. Moreover, any one of key telephones can be connected to the office line by operation of the office line selection key.
Such a type of key telephone system is well known in the prior art and is disclosed in, for example, U.S. Pat. No. 4,347,409 by Ogawa et al, assigned to Nippon Telegraph & Telephone Public Corporation and Nippon Tsushin Kogyo Kabushiki Kaisha.
In use of the key telephone system as established, addition of key telephone sets and/or office lines are limited by the permitted maximum number. This means that the user must replace the using system by a new system of a longer scale.
Moreover, when two similar key telephone systems are used at separate but near areas, these two key telephone systems cannot be connected to each other.